The present invention relates generally to implantable medical devices and more particularly to implantable medical devices intended for use in monitoring a patient""s heart rhythm.
Implantable pacemakers and cardioverters monitor the heart""s rhythm in order to detect arrhythmias and deliver appropriate therapies to terminate detected arrhythmias. In conjunction with this function, the ability of the device is to store information with regard to monitored heart rhythms has dramatically increased over the past two years. Examples of implantable pacemakers and defibrillators which have the capability of storing information related to monitor heart rhythms include U.S. Pat. No. 4,223,678 issued to Langer et al., U.S. Pat. No. 5,722,999 issued to Snell, U.S. Pat. No. 5,513,645 issued to Jacobsen et al. and U.S. Pat. No. 5,312,446 issued to Holschbach et al. In addition, there have recently been developed subcutaneously implantable monitoring devices which do not deliver any anti-arrhythmia therapies to the heart but simply store information regarding a patient""s heart rhythms for later uplink to an external device. Such devices are disclosed in U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 5,135,004 issued to Adams and U.S. Pat. No. 5,497,780 issued to Zehender.
In conjunction with implantable devices as described above, information stored relating to a patient""s heart rhythm may include information relating to heart rate trends over time, as disclosed as disclosed in U.S. Pat. No. 5,088,488 issued to Markowitz et al., U.S. Pat. No. 5,330,513 issued to Nichols et al. and U.S. Pat. No. 5,603,331 issued to Heemels et al., as well as information relating to heart rate variability over time, as disclosed in U.S. Pat. No. 5,749,900 issued to Schroeppel et. al., U.S. Pat. No. 5,466,245 issued to Spinelli et al., U.S. Pat. No. 5,411,131 issued to Yomtov et al. and U.S. Pat. No. 5,437,285 issued to Verrier et al.
Typically, measurements of heart rate variability in such devices is accomplished employing one of the standard measurements of heart rate variability such as the SDNN or SDANN indices, described in U.S. Pat. No. 5,749,900 issued to Schroeppel et al. and U.S. Pat. No. 5,411,031 issued to Yomtov, cited above. Unfortunately, obtaining clinically useful information employing the SDNN and SDANN indices traditionally requires the ability to correctly distinguish between normally conducted and ectopic beats, which may be difficult in some circumstances in the context of an implanted device.
The present invention is directed toward an implanted device having enhanced capabilities for monitoring a patient""s heart rate and heart rate variability trends over extended periods of time. The information collected by the implantable device is stored and telemetered to an associated external device such as a device programmer for display and analysis. The collected information and calculated heart rate variability may also be employed to trigger an alarm or alert to the patient, in response to a measured calculation of heart rate variabilty or change in heart rate variability. Heart rates are measured by measuring the time intervals between sensed depolarizations of a chamber of the patient""s heart and preceding sensed depolarizations or delivered pacing pulses. Intervals may be measured in the ventricle and/or atrium of the patient""s heart. The measured intervals are referred to hereafter as xe2x80x9cheart intervalsxe2x80x9d.
In a preferred embodiment of the invention, the device is configured to allow for measurements of heart rate variability trends over extended time periods, employing a measurement of rate variability which does not require the device to accurately distinguish between normally conducted and ectopic beats. In this aspect of the invention, rather than calculating the mean values only of heart intervals beginning and ending in normally conducted beats, the device instead calculates the medians of the stored intervals over short periods of time, including heart intervals beginning or ending in ectopic beats, and calculates the standard deviation of the median values. This difference in calculation methodology produces a measurement of heart rate variability (SDMNN) similar in accuracy to the SDANN index, but without the requirement of unambiguously identifying ectopic beats and eliminating the heart intervals associated therewith.
In addition, particularly valuable in the context of an implanted device, the process for calculation of a median interval is substantially simplified as opposed to the process for calculating the mean values of the measured intervals with ectopic beats and associated measured intervals excluded. In a preferred embodiment of the invention, the process of calculation of the median is further simplified by first producing a histogram of the stored heart interval values, sorting the intervals into bins each associated with an interval range. The numbers of intervals in each bin are then successively added to one another until the total exceeds half of the total number of intervals binned and the rate range associated with the last bin added is employed to define the median interval value.